In recent years, the use of on-demand digital printing machines utilizing electrophotographic systems has become widespread. In such systems, toner images are formed on a photosensitive drum and then electrically transferred onto an intermediate transfer blanket or belt for printing on a paper or polymeric film sheet or web. Toner formulations contain toner particles and solvents which keep the toner particles from becoming airborne. Conventionally, the solvents in these systems are removed either by evaporation prior to the transfer of toner to the substrate surface or by drying after the toner has been transferred. For example, a thermal offset blanket cylinder may be placed between the photosensitive image drum and the substrate surface which is supported on the backside by an impression drum. The toner is heated by the offset blanket cylinder to melt the toner into a uniform blend and to subsequently evaporate the solvent when the offset blanket cylinder transfers the toner onto the substrate surface.
Toner adhesion failure may result from one or more of various mechanisms. For example, the toner may fail to adhere to the substrate surface when there is no fusion step to fix the toner to the substrate surface. In another example, there may be an incomplete transfer of the toner to the substrate surface, leaving a residue on the offset blanket cylinder which translates into blanket life failure. Toners do not transfer well and/or adhere well to such films or paper substrates unless the substrates have been treated with a coating or primer (“optimized”) to enhance the adhesion of toners.
Primer coatings have been employed to promote adhesion of toners to paper or polymeric film substrates. Conventional ethylene-acrylic acid copolymer-based coatings used to enhance the adhesion of toner to paper and polymeric substrates often utilize an additional pre-coating.
Accordingly, there is a need in the art for a primer coating which provides toner adhesion to a variety of polymeric and non-polymeric substrates without the need for the application of additional pre-coatings, is stable, easy to handle, and meets the revised blanket compatibility requirement to achieve the highest rating.